Three-dimensional articles produced from CAD using layerwise rapid prototyping techniques (e.g. stereolithography) have been available for a number of years. The ultimate mechanical and other [e.g. surface finish etc] properties of such articles are derived from the materials and the CAD layering system being used.
For example, stereolithography systems, e.g. as described in U.S. Pat. No. 4,575,330, utilize complex photopolymer mixtures addressed by UV lasers to produce the 3 D articles. The polymer mixtures used in such machines have progressed from initial acrylic systems [e.g. U.S. Pat. No. 6,043,323], suitable for concept modeling, to epoxy-acrylic hybrids [e.g. U.S. Pat. Nos. 5,476,748, 6,136,497, 6,136,497, 6,100,007, 6,413,496, 6,099,787, 6,350,403, 5,437,964, 5,510,226, 5,494,618, 5,705,116] which have resulted in better properties [e.g. lower shrinkage, higher flexural modulus, better elongation at break, higher tensile properties, and higher impact resistance properties], usually however requiring compromises in one property or another. The improved performance from the epoxy-acrylic hybrids nevertheless has been useful for modeling certain technical properties.
There are alternative systems which yield layered three dimensional systems: Laser Sintering using thermoplastic powders, Extrusion Systems using extrudeable pastes, Jet Printing directly or onto Powders using fluids etc. All these material-machine systems have limitations in one area or another, which limit direct usefulness of the finally formed three-dimensional article. All of these systems also have limitations for speed of access to the formed three-dimensional articles: especially from those, which use laser scanning exposure processes.
There is continuing need to achieve, at higher speeds than hitherto possible, directly useful technical grade, three-dimensional articles.
Availability of such articles shows promise to open up the market, from service bureau type trade shops to ‘in house’ engineering design departments, and from rapid prototyping to short to medium volume rapid manufacturing.
In addition to these attributes, also highly desired are cured properties, which match or better those from thermoplastic polymers, used in conventionally produced articles [e.g. via injection moulds using polypropylene, ABS, polycarbonates, PEEK etc to name a few]
Further, there is a growing need for rapid prototyping materials and processes that produce three-dimensional articles, which are stable with time. In the past, articles produced by rapid prototyping were initially so brittle that increasing brittleness associated with aging went unnoticed. Oftentimes, the issue of brittleness was never confronted because articles were used only in short-term applications.
The present invention relates to photocurable compositions that can be used to produce three-dimensional articles, in a faster time frame and having superior and stable properties than hitherto possible.
Currently, the production of three-dimensional articles by selective irradiation of selected areas of successive layers of a photocurable composition uses a laser as the source of radiation since that provides a good speed of cure. Other radiation sources, which produce non-coherent radiation, cannot achieve the same curing speeds as laser radiation and require the use of much faster-curing compositions to provide sufficient green strength to enable the article to be self-supporting while, being built and before a final UV flood cure following removal from the bath in which it is built. However, the fast-curing polymers then to be brittle and to shrink substantially on curing, thereby degrading the accuracy of the model and causing parts of the model to curl.
Some prior art relating to thiolene type compositions are:
Example 1 of DE 4440819A1 describes a composition containing 76.1 g norbornen acrylate and 19.9 g polythiol which is cured under He/Cd lasers. The cured product has a high level of shrinkage: 14.4%. Cured products obtained from compositions containing about equivalent amounts by weight of norbornen acrylate and of polythiol have a lower shrinkage.
U.S. Pat. No. 4,230,740 describes compositions containing a polyene component and a thiol, typically together with a reactive diluent and a solvent which compositions are applied to a substrate and exposed to radiation, e.g. UV light, until crosslinking so as to provide non yellowing coatings on substrates. This is far from building 3 dimensional articles by optical moulding process.
EP0492953 A1 describes a stereolithography method for building a three-dimensional article using a curable resin formulation comprising compounds having plurality of norbornen groups, compounds comprising plurality of thiol groups and radical initiators. Typically similar ratios [e.g. 0.7:1 to 1.3:1] of the norbornyl compound and thiol compound are used. It is not suggested to produce 3D articles by non laser techniques. Norbornen compounds are smelly and their combination with smelly thiol compound is not recommended. Besides norbornen compounds have a low commercial availability.
U.S. Pat. No. 4,230,740 describes thiolene compositions containing allylcyanurate and thiol compounds, typically in similar ratios by weight. Usually heating for 15 minutes at 450 F is described to finish off the curing reaction.
Some prior art non-laser systems are:
WO 00/21735 describes a non-laser process but does not disclose that such a process should yield much improved mechanical properties from the resin.
U.S. Pat. No. 6,500,375 describes a three dimensional molding process involving spatial light modulators which regulate electro-magnetic irradiation falling on a resin. There is no mention that non-laser means of exposure can lead to improved properties.
Another possibility is that described in EP1250997A1 and EP1253002A1.
Another possibility is as described in US20040036200A1, US20040207123A1, WO 03041875, WO 03/099947 A1 and references therein which use jetting of UV curable fluids which are then cured by some non-laser irradiation.
Another possibility is use in direct printing e.g. U.S. Pat. No. 6,259,962 and related patents
This range of art generally discloses the machine/equipment method, but do not refer to the effect of the system on the ultimate properties attainable from such machines.
Polymers can be broadly classified as                thermoplastic: reform after heat to original state. Over and over again.        thermoset: set after heat. Only sets once. Can't reform        
Thermoplastics                crystalline: polymers arranged in a regular order        amorphous: polymers arranges randomly like coil        
Thermosets                low molecular weight monomers that crosslink and polymerize to for polymer network        
Elastomers                Can be either thermoplastic or thermoset                    Thermoset elastomers: natural and synthetic rubbers            Thermoplastic elastomers: plastics that mimic rubber (EPDK TPO, TPE)                        
The production of laser radiation requires costly equipment and it is desirable to find alternative ways to cure photocurable compositions. New photocurable compositions are needed for processes involving non-coherent actinic irradiation.